It has been known for many years to measure the flow rate (or volume) of a fluid flowing along a duct by using the propagation of sound signals transmitted between two sound transducers situated at points that are spaced apart in the flow direction of the fluid. Basically, a sound signal transmitted from the first transducer to the second transducer is received by the second transducer and the propagation time T.sub.1 of the sound signal is measured. Similarly, the propagation time T.sub.2 of a sound signal transmitted from the second transducer to the first transducer is measured after said signal has been received by said first transducer.
In a fluid meter, the flow rate can be obtained by combining the measured propagation times concerning the two sound signals transmitted in two opposite directions between the two points with a measurement of the sound phase shifts induced in each sound signal by the propagation of each of the sound signals in the flow. European patent application No. 0 426 309 describes an example of such a flow rate measuring system, in which the received signal is sampled and converted into digital form, with sound phase shift being measured by performing synchronous detection on the digitized signal.
Various types of electronic circuit can be used to meter fluid by the method of measuring the flow speed of the fluid by means of ultrasound. FIG. 4 is a block diagram of an example of such an electronic circuit associated with two ultrasound transducers 1, 2 located at a distance from each other in a duct 3 along which there flows a fluid such as a gas. The two transducers 1, 2 are connected to a switch block 4 which has two switches 5, 6 and enables each transducer to be used alternately as a transmitter and as a receiver. A transmitter module 14 and a receiver module 17 are respectively connected to the switches 5 and 6 in the switch block 4. The transmitter module 14 comprises an operational amplifier 16 and a digital-to-analog converter 15. The receiver module 17 comprise at least one amplifier 18 and an analog-to-digital converter 19 which digitizes and samples the received signal, e.g. simultaneously. An electrical power supply 7 and a power supply control module 8 are connected to the transmitter module 4 and the receiver module 17, and also to the switch block 4 and to an electronic control circuit such as a microcontroller 10. The microcontroller 10 includes, in particular, a crystal clock 9, an arithmetic and logic unit, random access memory (RAM), and read only memory (ROM), and is capable of co-operating with display circuits 13, a re-writable read only memory 12, and a serial link 11 of the RS 232 type.
Gas meters for each subscriber in a distribution network must be simultaneously accurate, reliable, and as cheap as possible. These constraints make it necessary to optimize the implementation of the electronic circuits and to avoid using expensive components such as inductors or operational amplifiers fitted with adjustment members for example, while nevertheless guaranteeing that there are no errors which could be due, for example, to the presence of stray capacitance or to zero-point drift associated with the fact that each transducer is not associated with the same impedance when operating as a transmitter and as a receiver.